Potentiometers find use in a variety of applications where a variable electrical resistance between input and output terminals is desired. Such uses include volume control, light control, instrumentation control, and the like. Most potentiometers include a housing in which a rotor turns, the rotor including electrical contacts that wipe across a resistive strip of a variable resister. In order for the rotor to be manipulated by the user, it must extend into the environment outside of the housing, thereby creating a path for the entry of corrosive elements into the interior of the housing where the electrical contacts of the potentiometer are placed. Because many potentiometers find an application in a potentially corrosive environment, this entry path into the housing is often sealed to prevent the intrusion by these externally originating, corrosion causing elements. In the application of the inventive potentiometer to be described below, that is, a hearing aid, such corrosive elements can include body fluids such as ear wax.
One known prior art potentiometer seal is the O-ring. This type of seal has typically been used to seal around a potentiometer rotor. For an O-ring seal to function effectively, however, the seal must be compressed such that it exerts a pressure against the rotor. Compressing an O-ring seal, however, affects its "running torque", that is the resistance offered by the O-ring to the rotor as it is turned therewithin. Compressing the O-ring seal, then, may make turning the rotor more difficult. A further problem with O-ring type seals are that they are relatively expensive. Yet another deficiency of an O-ring seal is that it is subject to quality variations that can effect the sealability of the seal. Since the seal must be compressed to function, the quality variations can effect the compressibility and thus the effectiveness of the O-ring as a seal. Still yet another deficiency of the O-ring seal typically found in a potentiometer is that the best pressure differential that such a seal can withstand is approximately 1/2 pound per square inch. Finally, the sealing integrity of the O-ring type of seal can actually diminish as the pressure differential from one side of the seal to the other side increases.
Another prior art type of seal used in certain potentiometer applications is the labyrinth seal. In that kind of application, a labyrinth seal typically comprises two or more concentric rings of different diameters that interleave with each other. Labyrinth seals are effective at keeping viscous fluids or particles out of the potentiometer housing, but such seals effectively wick thinner fluids into the housing. Most potentiometers have no inner seal other than the labyrinth seal; consequently, these thinner fluids can be wicked into corrosive contact with the electrical contacts of the variable resistor housed therein, leading to corrosion of the electrical contacts such that the potentiometer will cease to function. For example, in a potentiometer environment such as a hearing aid, a labyrinth seal may stop a thick, viscous fluid such as ear wax from entering the potentiometer housing, but may wick any thinner fluid that is present, such as sweat, regardless of the origin of the thinner fluid, into the housing where corrosion of the contacts may occur.
It would be desirable to have a potentiometer seal that would be less expensive than prior art seals; that would be less subject to manufacturing variations; that would not wick corrosive fluids into the housing; that would withstand greater pressure differentials; and that would have greater seal integrity as the pressure differential increased.